[RFC] UBSan unsafely uses VRP

[Marat Zakirov <m dot zakirov at samsung dot com>]

Hi all!

I found that UBSan uses vrp pass to optimize generated checks. Keeping 
in mind that vrp pass is about performance not stability I found example 
where UBSan may skip true positive.

Example came from spec2006 perlbench:

int ext;

int
Perl_do_sv_dump()
{
    int freq[10];
    int i;
    int max = 0;
    int t = INT_MAX - 20;

    if (max < ext)
      max = ext;

    for (i = 0; i <= max; i++)
        if (freq[i])
          ext = 0;

    t += i;  <<< (*)
    return t;
}

vrp pass here sets vrp('i') to [0..10] in assumption that 'freq[i]' wont 
violate array bound (vrp uses loop iteration number calculation, see 
adjust_range_with_scev in tree-vrp.c). This means that UBSAN_CHECK_ADD 
build for (*) should be deleted as redundant (and actually it is deleted 
by vrp pass). So if at the execution max = 30, freq[5] != 0 uncaught 
overflow will occur.

There are also some unsafe code in functions 
ubsan_expand_si_overflow_addsub_check, 
ubsan_expand_si_overflow_mul_check which uses get_range_info to reduce 
checks number. As seen before vrp usage for sanitizers may decrease 
quality of error detection.

Potentially unsafe vrp decisions should preferably not be exploited by 
any sanitizer pass. I have several ideas how we could achieve that:
1) Do not use vrp info for sanitizers.
2) Do not use unsafe conclusions during vrp pass if sanitizers are enabled.
3) Make two versions of value range data safe and unsafe.

P.S.

To reproduce the issue use appropriate GCC options 
(-fsanitize=signed-integer-overflow -S -O3) and attached trivial patch 
which disables UBSan check build in inner loops. This patch is needed 
because currently iteration number calculation used by vrp pass do not 
supports yet UBSAN_CHECK_[ADD/SUB/MUL].

--Marat

diff --git a/gcc/ubsan.c b/gcc/ubsan.c
index dde0418..97dbf33 100644
--- a/gcc/ubsan.c
+++ b/gcc/ubsan.c
@@ -1014,6 +1014,10 @@ instrument_si_overflow (gimple_stmt_iterator gsi)
       || GET_MODE_BITSIZE (TYPE_MODE (lhstype)) != TYPE_PRECISION (lhstype))
     return;
 
+  // skip for inner loops 
+  if (gimple_bb(stmt)->loop_father->inner == NULL)
+    return;
+
   switch (code)
     {
     case MINUS_EXPR: